Filtered Photons

Our backyard here in Florida has a lovely screened porch, and whenever I go out there with Lorna, I notice this interesting effect:

The pattern made by the two screens reminded me of an interference pattern, so familiar from quantum mechanics:

Wikipedia

This type of pattern is created by the electromagnetic waves from different sources cancelling each other out, but that's certainly not happening with our screen. Instead, the geometry of the situation leads to the gaps and wires of the mesh aligning in different ways. We can imagine looking through one screen at another moving horizontally:

As the wire from the moving screen passes through the gap of the stationary one, the amount of light is reduced, creating the darker regions seen in the photo. In this case, we're looking through the corner of two screens at right angles:

Because of the different distances between eye and screen, we end up in different points in the wire-gap pattern. I had trouble modeling those differing phases, and I suspect I've made an error somewhere, but the phase relation I get is

where the blue curve is the closer screen, and the red the more distant. That leads to a brightness pattern which looks like

While this does show brighter and darker patches, it seems to bear no resemblance to the type of pattern seen in quantum mechanics. I'm also not confident I have this correct, but I already spent too much time on debugging, so this will have to be another post with an unsatisfying conclusion!

Aunt Enna

We've still been busy moving in here in Florida, hence my long silence. Recently though we've been considering TV options, and I saw an opportunity to tie our search into a post! We'd prefer not to get cable, but we've been missing watching the morning news, and wondered if we could get it over broadcast. As is, our TV won't pick anything up, but the FCC says there should be some channels in our area. That made us consider getting an antenna to help reception.

I had a basic idea of how antennas worked – They transmit and receive electromagnetic waves, and they can be uni-directional, or omni-directional. What I wondered was how they achieve those qualities. It turns out there's a vast number of antenna designs, depending on what type of signal you want, whether you're sending or receiving, and how big it should be.

Before we get into that though, let's go over the principle of an antenna. Radio waves are a form of electromagnetic wave, meaning they consist of an oscillating electric and magnetic field. We want our antenna to translate between those fields and electric current. In the case of long, straight antennas, we use the electric field to do that:

As the electric field (red line) moves through the antenna (blue line) it makes the electrons in the antenna (blue dot) move up and down. This is an alternating current, which corresponds to the signal encoded in the wave.

Those classic rabbit-ear antennas have mostly been replaced by varieties of flat designs, like the one we're considering:

Amazon

I had hoped to get a straight answer on what's inside these, but it seems the designs vary between different models. They appear to be variations on the dipole antenna, which use two wires in opposite directions, but it works on the same principle as the long, straight antennas.

Before I found that teardown page, I was guessing they actually held small loop antennas. These are interesting, because rather than the electric part of the electromagnetic wave, they use the magnetic field to induce current in the loop. That means that the directions they're sensitive in are opposite those of an equivalent electric antenna.

Clearly there's been a lot of study put into designing antennas for different purposes, but with all the other stuff we have on our plate, we're going to try for the cheap one and hope for the best. I'll just be happy if it doesn't explode!